Joint detection-decoding receiver of ds-cdma system

ABSTRACT

A joint detection-decoding receiver using a multistage parallel interference canceller for selectively estimating a partial codeword corresponding to a state transition (branch) is disclosed. The JDD receiver of the DS-CDMA system includes a PIC that employs a state sequence to selectively carry out a symbol estimation of a partial codeword corresponding to the branch so as to reduce complexity and a calculation load as well as maintaining a high performance of the receiver according to the joint detection-decoding.

RELATED APPLICATIONS

The present disclosure relates to subject matter contained in priorityKorean Application No. 10-2006-0117360, filed on 27 Nov. 2006 which isherein expressly incorporated by reference in its entirety.

BACKGROUND OF THE INVENTION

The present invention relates to a joint detection-decoding receiver,and in particular, to a joint detection-decoding receiver using amultistage parallel interference canceller for selectively estimating apartial codeword corresponding to a state transition (branch).

A DS-CDMA (Direct Sequence Code Division Multiple Access) is widely usedas a multiple access scheme used in a wireless channel shared bymultiple users. In accordance with a DS-CDMA system, different signaturesequences or pseudo-noise (PN) codes are assigned to the multiple users.A DS-CDMA receiver comprises a bank of matched filters in order toobtain a user data from a received signal.

However, the DS-CDMA system has a disadvantage of a performancedegradation due to a MAI (multiple Access Interference) by other usersignals. In order to overcome the disadvantage, a method for removingthe MAI through a MUD (Multi-User Detection) has been proposed. The MUDmay reduce an effect of the MAI as well as improving a system capacityand a service quality. Considering an uplink of the DS-CDMA system, thereceiver of a base station has an information on a spreading sequence ofevery user. Therefore, the receiver may remove the MAI using theinformation.

A JDD (Joint Detection-Decoding) receiver is proposed as a novel methodfor detecting the multi-user. The JDD receiver utilizes an informationon a channel code to simultaneously carry out the MUD and a channeldecoding.

FIG. 1 is a block diagram illustrating a multi-user detectiontranceiving system of a DS-CDMA system.

Referring to FIG. 1, in accordance with the DS-CDMA system, a sequenceof information bits b_(k) of a user k is encoded into a sequence ofchannel symbols d_(k), and the sequence of channel symbols d_(k) istransmitted via a multi-user channel. The sequence of channel symbolsd_(k) is received at a receiver as an output y_(k) of a matched filterbank 30 through the matched filter bank 30. The output y_(k) not onlyincludes the MAI due to other users but also has a correlation over aframe of the channel code. Therefore, a multi-user detector 40 detects asymbol estimate {circumflex over (d)}_(k) for the other users from theoutput y_(k) of the matched filter bank, and a channel decoder 50calculates an information estimate {circumflex over (b)}_(k) of thesequence of information bits b_(k) through a decoding. An accuracy ofthe symbol estimation according to a channel encoding is improved and aneffect of the MAI is reduced by the joint detection-decoding. However,it is disadvantageous that a complexity of the JDD receiver is linearlyincreased proportional to a number of repetitions and compensations ofthe MUD and a frame size of the channel code.

A PIC (Parallel Interference Canceller) is generally used in the DS-CDMAsystem in order to effectively remove the MAI. The PIC predicts thesymbol for entire users simultaneously in parallel. A PIC receiverincluding attempts to remove the MAI through a signal processing of eachstage for the entire users. The PIC receiver has a much simplerstructure compared to a general linear MUI receiver.

FIG. 2 is a block diagram illustrating a parallel interference cancellerof a DS-CDMA system.

As shown, each stage of the PIC comprises estimators 101 and 201, andcancellers 102 and 202. The estimators 101 and 201 estimates the symbolsfor the entire users and the MAI for the entire users is removed basedon the estimation. A first stage of the PIC employs a bank of asingle-user matched filter. An accuracy of the prediction increasesaccording to a number of the stages, and a performance of the PIC isalso improved.

For the user k, when a current parallel interference cancellation stageis assumed to be l, the estimators 101 and 201 computes a symbolestimate {circumflex over (d)}_(k)(l|l−1) from the a posteriori signaly_(k)(l−1) of the previous stage. The symbol estimate {circumflex over(d)}_(k)(l|l−1) is treated as a priori information for a cancellationnext stage. The cancellers 102 and 202 removes the MAI included in auser signal y_(k)(l) using a correlation matrix R.

$\begin{matrix}{{y_{k}(l)} = {{y_{k}(0)} - {\sum\limits_{i \neq k}{\rho_{ik}{{\hat{d}}_{i}\left( {l\left. {l - 1} \right)} \right.}}}}} & \text{[Equation 1]}\end{matrix}$

A final decision is established based on the signal y_(k)(l) of a laststage. However, the estimators 101 and 201 of the conventional PICcarries out a hard-decision of the symbol estimate {circumflex over(d)}_(k)(l|l−1) through the posteriori signal y_(k)(l−1). When theestimators 101 and 201 make an improper decision, the interferenceremoval becomes inaccurate, thereby reducing the accuracy of theinterference removal is drastically reduced passing through each stage.Therefore, an accurate symbol estimation of the estimators 101 and 201is very important, and the JDD receiver improves the accuracy by usingthe channel code at the symbol estimation stage.

In accordance with the conventional JDD receiver, when a convolutioncode (n, m, l) having a code rate of m/n and a memory length of l isused as the channel code, an information sequence b_(k)=[b_(k) ⁽¹⁾, . .. ,b_(k) ^((N))]=[b_(k) ⁽¹⁾, . . . ,b_(k) ^((mN))] having a length of mNof a k-th user is encoded into a code word d_(k)=[d_(k) ⁽¹⁾, . . .,d_(k) ^((N+l))]=[d_(k) ⁽¹⁾, . . . ,d_(k) ^((n(N+l)))] having a lengthof n(N+1), where N is a frame size of a channel code.

Since the codeword is transmitted through a multi-user channel and anerror correction is possible by a redundant portion of the channel code,the JDD receiver improves the accuracy of the estimation stage. However,the entire codeword should be processed in the estimators 101 and 201 ofeach stage and the cancellers 102 and 202, the JDD receiver isdisadvantageous in that the complexity and a computation load of thereceiver are increased.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a JDD receiver of aDS-CDMA system wherein a PIC employs a state sequence to selectivelycarry out a symbol estimation of a partial codeword corresponding to abranch so as to effectively reduce complexity and a calculation loadcompared to the conventional JDD receiver wherein the symbol estimationis carried out for an entire codeword as well as maintaining a highperformance of the receiver according to a joint detection-decoding.

In order to achieve the above-described object, there is provided ajoint detection-decoding receiver of a DS-CDMA system, the receivercomprising: a matched filter for dividing a plurality of channel codedsymbols of a plurality of users according to each of the plurality ofusers to generate a matched filter output; and a multistage parallelinterference canceller for repeatedly carrying out a symbol estimationfor each of the plurality of users using the matched filter output and asymbol removal of other users using a correlation matrix to reduce amultiple access interference, wherein the multistage parallelinterference canceller estimates a partial code word corresponding to abranch as a symbol estimation using a state sequence.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram illustrating a conventional multi-userdetection tranceiving system of a DS-CDMA system.

FIG. 2 is a block diagram illustrating a conventional parallelinterference canceller of a DS-CDMA system.

FIG. 3 is a block diagram illustrating a multi-user detectiontranceiving system of a DS-CDMA system in accordance with the presentinvention.

FIG. 4 is a block diagram illustrating a parallel interference cancellerof a DS-CDMA system in accordance with the present invention.

FIG. 5 is a graph illustrating a complexity according to a number ofusers of a JDD receiver in accordance with the present invention.

FIG. 6 is a graph illustrating a complexity according to a number ofstages of a multi-stage parallel interference canceller of a JDDreceiver in accordance with the present invention.

FIGS. 7 thorough 9 are graphs illustrating a performance according avariation of a channel SNR of a JDD receiver under various communicationenvironments in accordance with the present invention.

FIGS. 10 thorough 12 are graphs illustrating a performance according anumber of users of a JDD receiver under various communicationenvironments in accordance with the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The above-described objects and other objects and characteristics andadvantages of the present invention will now be described in detail withreference to the accompanied drawings.

FIG. 3 is a block diagram illustrating a multi-user detectiontranceiving system of a DS-CDMA system in accordance with the presentinvention.

Referring to FIG. 3, a sequence of information bits b_(k) of a user k isencoded into a sequence of channel symbols d_(k), and the sequence ofchannel symbols d_(k) is transmitted via a multi-user channel. Thetransmitted sequence is received by the receiver of the presentinvention.

The JDD receiver in accordance with the present invention comprises amatched filter bank 30, a multi-user detector 40′ and a channel decoder50. The matched filter bank 30 and the channel decoder 50 are similar tothose of FIG. 1. Therefore, a detailed description is omitted.

The multi-user detector 40′ will now be described in detail.

FIG. 4 is a block diagram illustrating the parallel interferencecanceller of the DS-CDMA system in accordance with the presentinvention.

Referring to FIG. 4, the multi-user detector 40′ comprises estimators101′ and 201′ and cancellers 102′ and 202′. While only the estimators101′ and 201′ and the cancellers 102′ and 202′ are shown, the multi-userdetector 40′ comprises stage 1 through stage L, wherein each of thestages includes the estimator and the canceller.

The estimators 101′ and 201′ of the JDD receiver in accordance with thepresent invention employs Viterbi algorithm which is a MAP (maximum aposteriori) decoding to minimize a probability of a codeword error for aconvolution code. The JDD receiver in accordance with the presentinvention minimizes the probability of an error of a partial codewordconsisting of one or more channel symbols.

In a trellis diagram of the Viterbi algorithm, a path comprises a stateand a branch. Each entire codeword d_(k)=[d_(k) ⁽¹⁾, . . . ,d_(k)^((N+l))]=[d_(k) ⁽¹⁾, . . . ,d_(k) ^((n(N+l)))] may be represented by aunique state sequence S_(k)=[S_(k) ⁽⁰⁾, . . . ,S_(k) ^((t)), . . .,S_(k) ^((N+l+1))], where S_(k) ⁽⁰⁾=0 and vice versa. A partial codewordd_(k) ^((l))=[d_(k) ^(((n−1)(t+l)), . . . ,d_(k) ^((nt))] corresponds toa state transition (branch) from a state S_(k) ^((t−1)) at a time t−1 toa state S_(k) ^((t)) at a time t, where t=1, . . . ,N+l. Therefore, theentire codeword d_(k) of the channel symbols corresponds to the path ofthe convolution code and the partial codeword d_(k) ^((t)) of thechannel symbols corresponds to the branch of the convolution code of theconvolution code.

The partial codeword d_(k) ^((t)) also contains a partial decodinginformation about the transmitted symbols. The JDD receiver uses thepartial codeword d_(k) ^((t)) corresponding to the state transition(branch) in each of estimation steps, and constantly tracks the statesequence S_(k). Through these methods, a complexity of the receiver isreduced at a small expense of suboptimum decoding at the estimationsteps.

On the other hand, while a length of the partial codeword may selectedfrom one extreme of using a single transition sequence (minimum partialcodeword) to the other extreme utilizing the entire codeword, it ispreferable that the minimum partial codeword, i.e. the single transitionsequence is selected since the complexity increases as the length of thepartial codeword is increased despite an increase in a performanceimprovement.

The JDD receiver employs a posteriori signal y_(k) ^((t)) correspondingto the partial codeword d_(k) ^((t)) of the channel symbols. An estimate{circumflex over (d)}_(k) ^((t)) of the partial codeword is obtainedusing the posteriori signal y_(k) ^((t)) and a probability of an initialstate S_(k) ^((t−1)), and calculates a probability of final states S_(k)^((t)) for a next posteriori signal y_(k) ^((t+1)). A probabilityfunctions are defined as equations 2 and 3.

α_(t)(s _(t))=Pr{S _(k) ^((t)) =s _(t) ;y _(k) ⁽¹⁾ ; . . . ;y _(k)^((t))}  [Equation 2]

γ_(t)(s _(t−1) ,s _(t))=Pr{S _(k) ^((t)) =s _(t) ;y _(k) ^((t)) |S _(k)^((t−1)) =s _(t−1)}  [Equation 3]

where s_(t) and s_(t−1) are the states at the time t and the time t−1,respectively.

According to the Markov property of the convolution code, theprobability function of equation 2 may be expressed as equation 4.

$\begin{matrix}\begin{matrix}{{\alpha_{t}\left( s_{t} \right)} = {\sum\limits_{s_{t - 1}}{\Pr \left\{ {{{S_{k}^{({t - 1})} = s_{t - 1}};{S_{k}^{(t)} = s_{t}};y_{k}^{(1)}},\ldots \;,y_{k}^{(t)}} \right\}}}} \\{= {\sum\limits_{s_{t - 1}}{\Pr \left\{ {{{S_{k}^{({t - 1})} = s_{t - 1}};y_{k}^{(1)}},\ldots \;,y_{k}^{({t - 1})}} \right\} \Pr \left\{ S_{k}^{(t)} \right.}}} \\{{= s_{t}};{y_{k}^{(t)}\left. {S_{k}^{({t - 1})} = s_{t - 1}} \right\}}} \\{= {\sum\limits_{s_{t - 1}}{{\alpha_{t - 1}\left( s_{t - 1} \right)}{\gamma_{t}\left( {s_{t - 1},s_{t}} \right)}}}}\end{matrix} & \text{[Equation 4]}\end{matrix}$

Boundary conditions are as shown in equation 5.

α₀(0)=1, and α₀(i)=0 for i≠0.   [Equation 5]

Therefore, the probability function α_(t)(s_(t)) can be obtained fromthe probability function α_(t−1)(s_(t−1)) and the probability functionγ_(t)(s_(t−1),s_(t)) recursively. The probability functionγ_(t)(s_(t−1),s_(t)) of equation 3 may be expressed as equation 6.

$\begin{matrix}\begin{matrix}\left. {{\gamma_{t}\left( {s_{t - 1},s_{t}} \right)} = {{\Pr \left\{ {{S_{k}^{(t)} = s_{t}};y_{k}^{(t)}} \right.S_{k}^{({t - 1})}} = s_{t - 1}}} \right\} \\{= \frac{\Pr \left\{ {{S_{k}^{(t)} = s_{t}},{S_{k}^{({t - 1})} = s_{t - 1}},y_{k}^{(t)}} \right\}}{\Pr \left\{ {S_{k}^{({t - 1})} = s_{t - 1}} \right\}}} \\{{\left. {{= {{\Pr \left\{ y_{k}^{(t)} \right.S_{k}^{(t)}} = s_{t}}},{S_{k}^{({t - 1})} = s_{t - 1}}} \right\} \cdot \Pr}\left\{ S_{k}^{(t)} \right.} \\{= {s_{t}\left. {S_{k}^{({t - 1})} = s_{t - 1}} \right\}}} \\{= {\Pr \left\{ {y_{k}^{(t)}{\left. d_{k}^{(t)} \right\} \cdot \Pr}\left\{ {S_{k}^{(t)} = {s_{t}\left. {S_{k}^{({t - 1})} = s_{t - 1}} \right\}}} \right.} \right.}}\end{matrix} & \text{[Equation 6]}\end{matrix}$

Pr{S_(k) ^((t))=s_(l)|S_(k) ^((t−1))=s_(t−1)} is a state transitionprobability function defined as value of 1 when there is a trellistransition between the state s_(t−1) and s_(t), and as value of 0otherwise. Therefore, the probability function γ_(t)(s_(t−1),s_(t)) isequivalent to q branch metric in the Viterbi algorithm.

On the other hand, from equation 4, the MAP decoding on the statetransition (branch) becomes a procedure for maximizing a probability ofa next state based on α_(t−1)(s_(t−1)) and γ_(t)(s_(t−1),s_(t)) asexpressed in equation 7 below.

$\begin{matrix}{\begin{matrix}{{\overset{\hat{}\;}{S}}_{k}^{(t)} = {\arg \mspace{14mu} {\max\limits_{s_{t} \in S}\left\lbrack {\log \; {\Pr\left( {S_{k}^{(t)} = {s_{t}\left. {y_{k}^{(1)};\ldots \;;y_{k}^{(t)}} \right)}} \right\rbrack}} \right.}}} \\{{= {\arg \mspace{14mu} {\max\limits_{s_{t} \in S}\left\lbrack {\log \left\{ {{\Pr \left( {\alpha_{t}\left( s_{t} \right)} \right)} - {\Pr \left( {y_{k}^{(1)};\ldots \;;y_{k}^{(t)}} \right)}} \right\}} \right\rbrack}}},} \\{{{{{for}\mspace{14mu} t} = 1},\ldots \;,{N + 1},}} \\{= {\arg \mspace{14mu} {\max\limits_{s_{t} \in S}\left\lbrack {\log \; {\Pr \left( {\alpha_{t}\left( s_{t} \right)} \right)}} \right\rbrack}}}\end{matrix}\quad} & \text{[Equation 7]}\end{matrix}$

s is a set of the states, and s_(t) is the state at the time t ands_(t+1) is the state at the time t+1.

Finally, the estimate of the partial codeword {circumflex over (d)}_(k)^((t)) for t=1, . . . , N+1 may be expressed as equation 8.

$\begin{matrix}\begin{matrix}{{\hat{d}}_{k}^{(t)} = {\arg \mspace{14mu} {\max\limits_{{s_{t\;,}s_{t - 1}} \in S}\left\lbrack {\log \; {\Pr\left( {{S_{k}^{({t - 1})} = s_{t - 1}};S_{k}^{(t)}} \right.}} \right.}}} \\\left. {= {s_{t}\left. {y_{k}^{(1)};\ldots \;;y_{k}^{(t)}} \right)}} \right\rbrack \\{= {\arg \mspace{14mu} {\max\limits_{s_{t},{s_{t - 1} \in S}}\left\lbrack {\log \left\{ {{\Pr \left( {{\alpha_{t - 1}\left( s_{t - 1} \right)}{\gamma \left( {s_{t - 1},s_{t}} \right)}} \right)} -} \right.} \right.}}} \\\left. \left. {\Pr \left( {y_{k}^{(1)};\ldots \;;y_{k}^{(t)}} \right)} \right\} \right\rbrack \\{= {\arg \mspace{14mu} {\max\limits_{s_{t},{s_{t - 1} \in S}}\left\lbrack {\log \; {\Pr \left( {{\alpha_{t - 1}\left( s_{t - 1} \right)}{\gamma \left( {s_{t - 1},s_{t}} \right)}} \right)}} \right\rbrack}}}\end{matrix} & \text{[Equation 8]}\end{matrix}$

Once the estimate of the partial codeword {circumflex over (d)}_(k)^((t)) are obtained in the estimators 101 and 201 in each of the stages,the cancellers 102 and 202 of a corresponding stage calculates the MAIfor each of the users using the estimate of the partial codeword{circumflex over (d)}_(k) ^((t)) of each of the users and a correlationmatrix R to subtract the MAI calculated at an output y_(k) ^((t)) of thematched filter for each of the users. A new output signal is used as aninput of a prediction step in the next stage.

FIGS. 5 through 12 are graphs illustrating the performance and thecomplexity of the JDD receiver of the present invention, theconventional JDD receiver and a SDD (Separate Detection-Decoding)receiver.

FIGS. 5 and 6 are graphs illustrating the complexity of the receiversusing FLOP (FLoating Point Operations) according to an increase in anumber of users and stages in accordance with the present invention. Asshown, the conventional JDD employing the entire codeword is morecomplex than the SDD receiver. In particular, as shown in FIG. 6, theJDD receiver employing the partial codeword corresponding to the statetransition information in accordance with the present invention may beembodied to have a low complexity similar to the SDD receiver despitethe increases in the number of users and the PIC stages contrary to theconventional JDD receiver having the complexity that increasesdrastically according to the increase in the number of the users.

FIGS. 7 thorough 9 are graphs illustrating a performance using a BER(Bit Error Rate) according an increase in a channel SNR of the receiversin an environment of fifteen users, wherein a cross-correlation (rho) is0.1, 0.15 and 0.2. As shown, the JDD receiver in accordance with thepresent invention has a performance similar to the conventional JDDreceiver while the JDD receiver in accordance with the present inventionhas remarkably improved performance compared to the SDD receiver.

FIGS. 10 thorough 12 are graphs illustrating a performance according anumber of users of the receivers in an environment of 4 dB SNR, whereinthe cross-correlation (rho) is 0.1, 0.15 and 0.2. As shown, the JDDreceiver in accordance with the present invention has a performancesimilar to the conventional JDD receiver while the JDD receiver inaccordance with the present invention has remarkably improvedperformance compared to the SDD receiver.

As described above, in accordance with the JDD receiver of the DS-CDMAsystem of the present invention, the PIC employs the state sequence toselectively carry out the symbol estimation of the partial codewordcorresponding to the branch so as to effectively reduce complexity and acalculation load compared to the conventional JDD receiver wherein thesymbol estimation is carried out for the entire codeword as well asmaintaining a high performance of the receiver according to the jointdetection-decoding.

While the present invention has been particularly shown and describedwith reference to the preferred embodiment thereof, it will beunderstood by those skilled in the art that various changes in form anddetails may be effected therein without departing from the spirit andscope of the invention as defined by the appended claims.

1. A joint detection-decoding receiver of a DS-CDMA system, the receivercomprising: a matched filter for dividing a plurality of channel codedsymbols of a plurality of users according to each of the plurality ofusers to generate a matched filter output; and a multistage parallelinterference canceller for repeatedly carrying out a symbol estimationfor each of the plurality of users using the matched filter output and asymbol removal of other users using a correlation matrix to reduce amultiple access interference, wherein the multistage parallelinterference canceller estimates a partial code word corresponding to abranch as a symbol estimation using a state sequence.
 2. The receiver inaccordance with claim 1, wherein the plurality of symbols are encodedusing a convolution code, and wherein the receiver decodes the pluralityof the symbols using a MAP decoding.
 3. The receiver in accordance withclaim 2, wherein an information sequence b_(k)=[b_(k) ⁽¹⁾, . . . ,b_(k)^((N))]=[b_(k) ⁽¹⁾, . . . ,b_(k) ^((mN))] having a length of mN of ak-th user is encoded into a code word d_(k)=[d_(k) ⁽¹⁾, . . . ,d_(k)^(N+1))]=[d_(k) ¹⁾, . . . ,d_(k) ^((n(N+1)))] having a length of n(N+1)using a convolution code (n, m, l) having a code rate of m/n and amemory length of l, where N is a frame size of a channel code.
 4. Thereceiver in accordance with one claims 1 and 2, wherein the multistageparallel interference canceller selectively estimates the partial codeword of a section corresponding to a single state transition.
 5. Thereceiver in accordance with claim 4, wherein the multistage parallelinterference canceller estimates the partial code using an equation${{\hat{d}}_{k}^{(t)} = {\arg \mspace{11mu} {\max\limits_{s_{t},{s_{t - 1} \in S}}\left\lbrack {\log \; {\Pr \left( {{\alpha_{t - 1}\left( s_{t - 1} \right)}{\gamma \left( {s_{t - 1},s_{t}} \right)}} \right)}} \right\rbrack}}},$wherein α_(t)(s_(t)) is defined as${{\alpha_{t}\left( s_{t} \right)} = {\sum\limits_{s_{t - 1}}{{\alpha_{t - 1}\left( s_{t - 1} \right)}{\gamma_{t}\left( {s_{t - 1},s_{t}} \right)}}}},$α₀(0)=1, and α₀(i)=0 for i≠0, and γ_(t)(s_(t−1)s_(t)) is defined asγ_(t)(s_(t−1),s_(t))=Pr{y_(k) ^((t))|d_(k) ^((t))}·Pr{S_(k)^((t))=s_(t)=|S_(k) ^((t−1))=s_(t−1)}, where S_(t) is a state at a timet, and S_(t−l) is a state at a time t−l.